QP1 Flashcards
(26 cards)
What is a black body?
A black body is an idealisation, it is a perfect absorber and perfect emitter of radiation
The blackbody spectrum properties:
- ) the total intensity I is given by the Stefan Boltzmann law: I=rhoT^4
- )The intensity is not uniformly distributed over all wavelengths. Its distribution is measured by the spectral emittance I(lambda), the intensity per wavelength interval. The peak is given by the Wien displacement law: LambaT
- )The shape of the distribution is the same for all temperatures.
Rayleigh-Jeans law properties
- )Agrees with experiment at long wavelengths
- )Fails to reproduce short wavelength behaviour
- )Predicts an intensity per wavelength interval that tends to infinity at small wavelength- the ultraviolet catastrophe
Planck’s hypothesis introduces the assumption:
that an oscillator with frequency f can only have energies given by: E=nhf, where n is a positive integer. See slides for Planck’s radiation law
What is ultraviolet catastrophe?
Classical calculations predicted infinite total intensity - Rayleigh Jeans.
How did Planck solve the ultraviolet catastrophe?
He allowed oscillators to have only certain, quantized values of energy
For short wavelengths, almost all the oscillators are in the:
ground state, where they cannot emit radiation.
What is the photo-electric effect?
the emission of electron from a surface when light shines on it
What is needed to eject an electron from a surface?
To eject an electron the light must supply enough energy to overcome the forces holding the electron in the material.
What is the work function of a material?
the amount of energy an electron needs to escape the surface
The maximum kinetic energy of an electron is given by the excess energy:
eV0=hf-phi
Photon momentum:
Light has a momentum: p=E/c=hf/c=h/lambda
How were X-rays produced?
They were produced as bremsstrahlung when electrons slowed abruptly
What is Compton scattering?
Wave and particle models give different predictions when light is scattered off a single electron, such as an electron in an atom. The full predictions may be calculated using conservation of energy and momentum for the photon-electron system, and we obtain: deltalambda = wavelength of scattered light - wavelength of incident light = h/mc(1-cosphi)
Review slides 4
Heisenberg uncertainty principle and wave length dual.
The emission spectrum for solid bodies is:
continuous
The emission spectrum for gases is:
discrete
What is important about Rutherford’s experiment
The back scattering experienced by the Alpha particles.
The Bohr model important points:
- ) Atoms have specific allowed energy levels.
- ) Atoms are stable and therefore must have a ground level.
- )Each wavelength in the spectrum corresponds to a transition between two specific energy levels: E=hf=Ei-Ef
- )Bohr’s model related spectra to energy level structure, but didn’t give a way to predict the energy levels of a particular atom.
Lowest energy level - ground level- corresponds to n=
1
For energy needed to jump energy levels use equation:
En = -13.61/n^2eV
The centripetal force is provided by the:
Coulomb force, and quantisation of angular momentum.
In the quantum case, the derivation of the wave function physically represent the momentum, and the second derivative the:
Kinetic energy, thus we require the wave function and its derivative to be continuous.
Quantum particles have wave-like properties,
and are described by a wave function.
